Fredrix - Stretcher Strip - Heavy Duty - 18in - Fredrix Heavy Duty Stretcher Strips are made from kiln dried SPF grade wood and feature the unique Fredrix interlocking tongue and groove design, which was first developed in 1875. Each stretcher strip measures 2-1/4 + 1-7/16 inches with a full 1/2 inch tapered lip for maximum clearance. They are molded on modern woodworking machines to ensure a smooth and tight fit.